MD5使用接口
NSString+Hash.h

//
//  NSString+Hash.h
//
//  Created by Tom Corwine on 5/30/12..
//

#import <Foundation/Foundation.h>

@interface NSString (Hash)

@property (readonly) NSString *md5String;
@property (readonly) NSString *sha1String;
@property (readonly) NSString *sha256String;
@property (readonly) NSString *sha512String;

- (NSString *)hmacSHA1StringWithKey:(NSString *)key;
- (NSString *)hmacSHA256StringWithKey:(NSString *)key;
- (NSString *)hmacSHA512StringWithKey:(NSString *)key;

@end

MD5使用
ViewController.m

#import "ViewController.h"

#import "NSString+Hash.h"

#define Salt @"fsdhjkfhjksdhjkfjhkd546783765"

@implementation ViewController

- (void)viewDidLoad{
    [super viewDidLoad];
    
    [self digest4:@"123"];
    [self digest4:@"abc"];
    [self digest4:@"456"];
}

/**
 *  直接用MD5加密
 */
- (NSString *)digest:(NSString *)str{
    NSString *anwen = [str md5String];
    NSLog(@"%@ - %@", str, anwen);
    return anwen;
}

/**
 *  加盐
 */
- (NSString *)digest2:(NSString *)str{
    str = [str stringByAppendingString:Salt];
    NSString *anwen = [str md5String];
    NSLog(@"%@ - %@", str, anwen);
    return anwen;
}

/**
 *  多次MD5
 */
- (NSString *)digest3:(NSString *)str{
    NSString *anwen = [str md5String];
    anwen = [anwen md5String];
    NSLog(@"%@ - %@", str, anwen);
    return anwen;
}

/**
 *  先加密, 后乱序
 */
- (NSString *)digest4:(NSString *)str{
    NSString *anwen = [str md5String];
    NSString *header = [anwen substringToIndex:2];
    NSString *footer = [anwen substringFromIndex:2];
    anwen = [footer stringByAppendingString:header];
    NSLog(@"%@ - %@", str, anwen);
    return anwen;
}

@end